1. Field of the Invention
The present invention relates to a control system for controlling valve timing and the like of intake valves and exhaust valves of an internal combustion engine, during starting of the engine and catalyst warmup control after the start of the engine.
2. Prior Art
Conventionally, a control system for controlling valve timing of intake valves of an internal combustion engine has been proposed e.g. in Japanese Laid-Open Patent Publication (Kokai) No. H11-153075 (page 15, FIGS. 4 and 17). This engine includes a variable cam phase mechanism that changes the relative phase between each intake cam and the associated intake camshaft to thereby change the valve-closing timing and valve-opening timing of the intake valves, i.e. the valve timing of the intake valves, while maintaining the same valve open time over which each intake valve is open. This control system advances the valve timing of the intake valves at the start of the engine, using the variable cam phase mechanism, whereby the valve overlap is increased. This is to decrease the effective compression volume of a mixture to decrease load on the motor as a starter device, and thereby enhance the startability of the engine.
Further, a control system for an internal combustion engine for controlling ignition timing of the engine has been proposed e.g. in Japanese Laid-Open Patent Publication (Kokai) No. H08-218995 (page 8, FIG. 6). This control system carries out catalyst warmup control after the start of the engine for the purpose of reducing exhaust emissions by early activation of a catalytic converter. More specifically, the amount of retardation of ignition timing is calculated according to the catalyst temperature, and the ignition timing is retarded according to the calculated retardation amount. This enables early activation of the catalytic converter and thereby attain reduction of exhaust emissions.
According to the control system disclosed in Japanese Laid-Open Patent Publication (Kokai) No. H11-153075, the valve timing of intake valves is advanced at the start of the engine, so that the valve-closing timing of each intake valve is advanced. Therefore, when the intake temperature or the engine temperature is low, the lowering of the temperature within the cylinder caused by adiabatic expansion of the mixture within the cylinder can cause liquidation of fuel, resulting in degraded startability and increased exhaust emissions.
According to the control system disclosed in Japanese Laid-Open Patent Publication (Kokai) No. H08-218995, the ignition timing retard control is carried out after the start of the engine, with a view to attaining early activation of the catalytic converter for reduction of exhaust emissions. However, the effects of reduction of exhaust emissions by this control are limited, and particularly when a fuel having poor vaporizing combustion properties is used, it is impossible to increase the set value of the retardation amount of ignition timing due to instability of the combustion state of the engine, so that it takes much time before the catalytic converter is activated and excellent reduction of exhaust emissions cannot be attained in the meantime.